Recovery Biomarkers
Recovery biomarkers are used as referent measures to assess the validity of
self-reported intake. The correlation between recovery biomarkers and intake is
generally greater than 0.80. These biomarkers require the sampling of biologically
excreted, nutrient-related end products that reflect the actual consumption of the
nutrient under study. Unfortunately, only a few recovery biomarkers are available
for nutritional research; some are costly, some are complex to collect and analyze,
and the recovery generally requires a significant commitment from the individual
under study in order to collect a minimum single 24-h urine sample. The currently
employed recovery biomarkers include doubly labeled water (energy), urinary
nitrogen (proNitrogentein), and urinary potassium.
Doubly Labeled Water
Doubly labeled water (DLW) methodology is used to estimate energy intake in
free-living individuals. The method includes the administration of two stable iso-
topes of oxygen and hydrogen, followed by measurement of isotope elimination
curves as a measure of carbon dioxide production and thus the method indirectly
serves as an estimate of energy expenditure (Speakman 1998 ). The fundamental
premise of DLW methodology is that oxygen turnover is dominated by waterflow
through the body in combination with inspired O 2 and expired CO 2. The method
requires subjects to ingest the DLW and to collect 24-h urine samples just prior to
and 2 weeks following ingestion. The method has been widely accepted as the gold
standard of“true”energy intake and has been used to determine the energy uti-
lization associated with illness, physical exertion, and weight change (Schoeller
2008 ; Westerterp and Goris 2002 ). Alternately, the Goldberg cutoff, which com-
pares energy intake to basal metabolic rate, can be used to estimate energy
expenditure and in turn has been effectively applied to identify underreporting in
study populations (Livingstone and Black 2003 ).
Urinary Nitrogen
Urinary nitrogen is quite possibly the most well-known biomarker for assessing
protein intake, having been described by Otto Folin as early as 1905. Urinary
nitrogen is used in nitrogen balance studies as well as used as a tool for validating
protein intake. In a meta-analysis by Kipnis et al. ( 2001 ), urinary nitrogen was
reported to represent approximately 80% of dietary protein intake. Due to daily
variation, 1 day of urine collection is not adequate and therefore measurement of
intake and output is required over several days in order to reliably depict the
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